Electroless copper plating solution

ABSTRACT

The invention relates to an electroless aqueous copper plating solution, comprising: a source of copper ions, a reducing agent or a source of a reducing agent, and a combination of complexing agents comprising i) polyamino disuccinic acid, polyamino monosuccinic acid, or a combination thereof, and ii) one or more of ethylenediamine tetraacetic acid, N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid, and N,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine, as well as methods for electroless copper plating utilizing the solution and uses of the solution for the plating of various substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase application of InternationalPatent Application Number PCT/EP2014/055962, filed Mar. 25, 2014 whichclaims the benefit of European Patent Application Number EP 13161330.9,filed Mar. 27, 2013. The contents of each of these applications ishereby incorporated by reference in their entireties.

The present invention relates to an electroless copper plating solution,a method for electroless copper plating utilizing said solution and theuse of the solution for the plating of substrates.

Electroless plating is the controlled autocatalytic deposition of acontinuous film of metal without the assistance of an external supply ofelectrons. Non-metallic surfaces may be pretreated to make themreceptive or catalytic for deposition. All or selected portions of asurface may suitably be pretreated. The main components of electrolesscopper baths are the copper salt, a complexing agent, a reducing agent,and, as optional ingredients, an alkaline agent, and additives, as forexample stabilizers. Complexing agents are used to chelate the copper tobe deposited and prevent the copper from being precipitated fromsolution (i.e. as the hydroxide and the like). Chelating copper rendersthe copper available to the reducing agent which converts the copperions to metallic form.

U.S. Pat. No. 4,617,205 discloses a composition for electrolessdeposition of copper, comprising copper ions, glyoxylate as reducingagent, and a complexing agent, for example EDTA, which is capable offorming a complex with copper that is stronger than a copper oxalatecomplex.

U.S. Pat. No. 7,220,296 teaches an electroless plating bath comprising awater soluble copper compound, glyoxylic acid and a complexing agentwhich may be EDTA.

US 20020064592 discloses an electroless bath comprising a source ofcopper ions, glyoxylic acid or formaldehyde as reducing agent, and EDTA,tartrate or alkanol amine as complexing agent.

US 20080223253 discloses an electroless copper plating solutionincluding a copper salt, a reductant that may be selected from the groupconsisting of formaldehyde, paraformaldehyde, glyoxylic acid, NaBH₄,KBH₄, NaH₂PO₂, hydrazine, formalin, polysaccharide, such as glucose, anda mixture thereof, and a complexing agent which may be selected from thegroup consisting of ethylenediamine tetraacetic acid (EDTA),N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid (HEDTA),cyclohexanediamine tetraacetic acid, diethylenetriamine pentaaceticacid, and N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol).

Performance of a copper plating solution is difficult to predict andstrongly depends on its constituents, especially the complexing agentand the reducing agent, and the molar ratio of its constituents.

An object of the invention was to provide with an electroless copperplating solution with improved performance, particularly an improvedcopper deposition rate.

The present invention provides with an electroless copper platingsolution, comprising

-   -   a source of copper ions,    -   a reducing agent or a source of a reducing agent, and    -   a combination comprising    -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) one or more of a compound which is selected from        ethylenediamine tetraacetic acid,        N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid, and        N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine,    -   as complexing agents.

Ethylenediamine tetraacetic acid is hereinafter also named as “EDTA”.

N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid is hereinafteralso named as “HEDTA”.

N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine is hereinafter alsonamed as “Quadrol”, which is a trademark of BASF company.

One or more of the above mentioned objects are achieved by theelectroless copper plating solution (hereinafter abbreviated as the“solution”) according to the above electroless copper plating solution,or by advantageous embodiments as described in dependent claims and thedescription. The copper plating solution of the invention shows animproved copper deposition rate. At the same time, a low roughness ofcopper surfaces can be reached, which is crucial for performance ofcertain electronic devices. Due to a higher deposition rate, a higherthickness of copper layer can be reached at the same process time.

The solution according to the invention and the process according to theinvention are preferably used for the coating of printed circuit boards,chip carriers and semiconductor wafers or also of any other circuitcarriers and interconnect devices. The solution is used in particular inprinted circuit boards and chip carriers, but also in semiconductorwafers, to plate surfaces, trenches, blind micro vias, through hole vias(through holes) and similar structures with copper.

Particularly, the solution of the invention or the process of theinvention can be used for deposition of copper on surfaces, in trenches,blind-micro-vias, through-hole-vias, and comparable structures inprinted circuit boards, chips, carriers, wafers and various otherinterconnect devices. The term “through hole vias” or “through holes”,as used in the present invention, encompasses all kinds of through holevias and includes so-called “through silicon vias” in silicon wafers.

Another application wherein the solution can be used with beneficialeffects is metallization of smooth, substrates made from glass, ceramicor plastics, preferably with a large surface area. Examples are any kindof displays, as for example any kind of TFT-displays and liquid crystaldisplays (LCD). As mentioned above, a low roughness of copper surfacescan be reached with the solution of the invention. This effect isadvantageous particularly for display applications, since a copper layerhaving a low thickness, which means a thickness that is suitable forsuch applications, and having a good conductivity, can be produced. Theroughness of a copper surface, produced with the solution of theinvention, is preferably 5-40 nm, more preferably 10-30 nm, still morepreferably 15-30 nm, expressed as root-mean-square roughness parameter(RMS). Explanation of the method for roughness measurement and theroot-mean-square roughness parameter (RMS) is given in the examples. Theinvention in one aspect relates also to a copper-plated article, havinga copper layer with a roughness of 5-40 nm, more preferably 10-30 nm,still more preferably 15-30 nm, expressed as root-mean-square roughnessparameter (RMS).

The electroless copper plating solution of the invention can bebeneficially used for deposition of copper on a glass substrate,particularly with large surface area, such as glass panels. Glasssubstrates are used, without limitation, for display applications, asmentioned above. Wet electroless copper deposition, with a solution asmentioned above, on a glass substrate is beneficial in comparison tometal sputtering processes that have been used so far. Benefits that canbe reached with wet electroless deposition in comparison to sputteringtechniques are, inter alia, reduced internal stress and reduced bendingof a glass substrate, reduced equipment maintenance, effective use ofmetal, reduced material waste, reduced process temperature.

Anyhow, common wet electroless deposition usually produces rougher metalsurfaces than sputtering processes. In the case of display productionthis causes poor switching properties, especially unfavorable prolongedswitching times. Thus, for display production it is necessary togenerate metal layers with a roughness in the range achieved bysputtering processes. Surprisingly the electroless copper platingsolution of the invention is not only able to generate metal layers athigher deposition rate, but simultaneously with a low roughness in arange achieved by sputtering processes.

Moreover, substrates for display production are activated by metal seedlayers for subsequent deposition of metal layers in order to buildnecessary circuitry and switching elements. Thus, the metal seed layersalready display the future pattern of circuitry and switching elementsthat comprise small and/or isolated activated areas as well as acombination of small and larger activated areas. A high copperdeposition rate on glass substrate is reached with the solution of theinvention, especially on glass substrates that have these small and/orisolated activated areas. In addition the solution of the invention alsois able to deposit metal layers with uniform thickness simultaneouslyonto small and larger activated areas at high deposition rates.

The solution of the invention is an aqueous solution. The term “aqueoussolution” means that the prevailing liquid medium, which is the solventin the solution, is water. Further liquids, that are miscible withwater, as for example alcohols and other polar organic liquids, may beadded.

The solution of the present invention may be prepared by dissolving allcomponents in aqueous liquid medium, preferably in water.

The solution contains a copper ion source, which may for example be anywater soluble copper salt. Copper may for example, and withoutlimitation, be added as copper sulphate, copper chloride, coppernitrate, copper acetate, copper methane sulfonate ((CH₃O₃S)₂Cu), copperhydroxide; or hydrates thereof.

The reducing agent serves for reducing the copper ions in order toobtain metallic copper for plating. Reducing agents that can be employedare for example, and without limitation, formaldehyde, glyoxylic acid,hypophosphite, hydrazine, and borohydride. Preferred reducing agents areformaldehyde and glyoxylic acid.

The term “source of a reducing agent” means a substance that isconverted to a reducing agent in the solution. The source is for examplea precursor of a reducing agent that converted to the reducing agent. Anexample is given below with respect to glyoxylic acid.

A particularly preferred reducing agent is glyoxylic acid because ofsafety, health and environmental requirements. Even though formaldehydeis a very important and established reducing agent of the commonelectroless copper plating process, it was classified as a probablehuman carcinogen. Thus, the electroless aqueous copper plating solutionin one embodiment comprises glyoxylic acid or a source of glyoxylicacid. In this embodiment, the solution of the invention does not containformaldehyde, or, in other words, the solution is according to thisembodiment free of formaldehyde.

The term “source of glyoxylic acid” encompasses all compounds that canbe converted to glyoxylic acid in aqueous solution, such as precursors.A preferred precursor is dichloro acetic acid. Glyoxylic acid is thereducing agent for the reduction of copper ions to elementary copper. Inthe solution, glyoxylic acid and glyoxylate-ions may be present. As usedherein the term “glyoxylic acid” includes salts thereof. The exactnature of the species, acid or salt, present will depend on the pH ofthe solution. The same consideration applies to other weak acids andbases.

In addition to one of the above-mentioned reducing agents, one or moreadditional reducing agents may be added, as for example hypophosphoricacid, glycolic acid or formic acid, or salts of aforementioned acids.The additional reducing agent is preferably an agent that acts asreducing agent but cannot be used as the sole reducing agent (cf. forexample the disclosure in U.S. Pat. No. 7,220,296, col. 4, I. 20-43 and54-62). Therefore, such additional reducing agent is in this sense alsocalled an “enhancer”.

Electroless copper baths using reducing agents described abovepreferably employ a relatively high pH, usually between 11 and 14,preferably between 12.5 and 13.5, and are adjusted generally bypotassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide(LiOH), ammonium hydroxide or quarternary ammonium hydroxide, such astetramethylammonium hydroxide (TMAH). Thus, the solution may contain asource of hydroxide ions, as for example and without limitation one ormore of the compounds listed above. A source of hydroxide is for exampleadded if an alkaline pH of the solution is desired and if the pH is notalready in the alkaline range by other constituents.

Especially preferred is the use of potassium hydroxide. Potassiumhydroxide is of advantage, if glyoxylic acid is used as the reducingagent, because the solubility if potassium oxalate is high. Oxalateanions are formed in the solution by the oxidation of the glyoxylicacid. Thus, potassium hydroxide is especially preferable for stabilityof the solution of the present invention.

Polyamino disuccinic acids are compounds having two or more nitrogenatoms, wherein 2 of the nitrogens are bonded to a succinic acid (orsalt) group, preferably only two nitrogen atoms each have one succinicacid (or salt) group attached thereto. As used herein the term succinicacid includes salts thereof. The compound has at least 2 nitrogen atoms,and due to the commercial availability of the amine, preferably has nomore than about 10 nitrogen atoms, more preferably no more than about 6,most preferably 2 nitrogen atoms. Nitrogen atoms which do not have asuccinic acid moiety attached most preferably are substituted withhydrogen atoms. More preferably, the succinic acid groups are onterminal nitrogen atoms, most preferably each of which nitrogens alsohave a hydrogen substituent. By terminal it is meant the first or lastnitrogen atom which is present in the compound, irrespective of othersubstituents. Another definition of a terminal nitrogen is a primaryamine nitrogen, before a succinic acid moiety is attached. The terminalnitrogen is transferred to a secondary amine nitrogen after a succinicacid moiety was attached. Because of steric hindrance of two succinicgroups on one nitrogen, it is preferred that each nitrogen having asuccinic group has only one such group. Remaining bonds on nitrogenshaving a succinic acid group are preferably filled by hydrogens or alkylor alkylene groups (linear, branched or cyclic including cyclicstructures joining more than one nitrogen atom or more than one bond ofa single nitrogen atom, preferably linear) or such groups having etheror thioether linkages, all of preferably from 1 to 10 carbon atoms, morepreferably from 1 to 6, most preferably from 1 to 3 carbon atoms, butmost preferably hydrogen. More preferably, the nitrogen atoms are linkedby alkylene groups, preferably each of from 2 to 12 carbon atoms, morepreferably from 2 to 10 carbon atoms, even more preferably from 2 to 8,most preferably from 2 to 6 carbon atoms, namely ethylene, propylene,butylene, pentylene or hexylene. The polyamino disuccinic acid compoundpreferably has at least about 10 carbon atoms and preferably has at mostabout 50, more preferably at most about 40, most preferably at mostabout 30 carbon atoms. The term “succinic acid” is used herein for theacid and salts thereof; the salts include metal cation (e.g. potassium,sodium) and ammonium or amine salts.

Polyamino disuccinic acids useful in the practice of the invention areunsubstituted (preferably) or inertly substituted, that is substitutedwith groups that do not undesirably interfere with the activity of thepolyamino disuccinic acid in a selected application. Such inertsubstituents include alkyl groups (preferably of from 1 to 6 carbonatoms); aryl groups including arylalkyl and alkylaryl groups (preferablyof from 6 to 12 carbon atoms), with alkyl groups preferred among theseand methyl and ethyl groups preferred among alkyl groups.

Inert substituents are suitably on any portion of the molecule,preferably on carbon atoms, more preferably on alkylene groups, forexample alkylene groups between nitrogen atoms or between carboxylicacid groups, most preferably on alkylene groups between nitrogen groups.

Preferred polyamino disuccinic acids includeethylenediamine-N,N′-disuccinic acid (EDDS),diethylenetriamine-N,N″-disuccinic acid,triethylenetetraamine-N,N′″-disuccinic acid, 1,6 hexamethylenediamineN,N′-disuccinic acid, tetraethylenepentamine-N,N′″-disuccinic acid,2-hydroxypropylene-1,3-diamine-N,N′-disuccinic acid, 1,2propylenediamine-N,N′-disuccinic acid,1,3-propylenediamine-N,N″-disuccinic acid,cis-cyclohexanediamine-N,N′-disuccinic acid,transcyclohexanediamine-N,N′-disuccinic acid, andethylenebis(oxyethylenenitrilo)-N,N′-disuccinic acid. The preferredpolyamino disuccinic acid is ethylenediamine-N,N′-disuccinic acid.

Such polyamino disuccinic acids can be prepared, for instance, by theprocess disclosed by Kezerian et al. in U.S. Pat. No. 3,158,635 which isincorporated herein by reference in its entirety. Kezerian et aldisclose reacting maleic anhydride (or ester or salt) with a polyaminecorresponding to the desired polyamino disuccinic acid under alkalineconditions. The reaction yields a number of optical isomers, forexample, the reaction of ethylenediamine with maleic anhydride yields amixture of three optical isomers [R,R], [S,S] and [S,R] ethylenediaminedisuccinic acid (EDDS) because there are two asymmetric carbon atoms inethylenediamine disuccinic acid. These mixtures are used as mixtures oralternatively separated by means within the state of the art to obtainthe desired isomer(s). Alternatively, [S,S] isomers are prepared byreaction of such acids as L-aspartic acid with such compounds as1,2-dibromoethane as described by Neal and Rose, “Stereospecific Ligandsand Their Complexes of Ethylenediaminedisuccinic Acid”, InorganicChemistry, V. 7, (1968), pp. 2405-2412.

Polyamino monosuccinic acids are compounds having at least two nitrogenatoms to which a succinic acid (or salt) moiety is attached to one ofthe nitrogen atoms. Preferably the compound has at least 2 nitrogenatoms, and due to the commercial availability of the amine, preferablyhas no more than about 10 nitrogen atoms, more preferably no more thanabout 6, most preferably 2 nitrogen atoms. Remaining nitrogen atoms,those which do not have a succinic acid moiety attached, preferably aresubstituted with hydrogen atoms. Although the succinic acid moiety maybe attached to any of the amines, preferably the succinic acid group isattached to a terminal nitrogen atom. By terminal it is meant the firstor last amine which is present in the compound, irrespective of othersubstituents. Another definition of a terminal nitrogen is a primaryamine nitrogen, before a succinic acid moiety is attached. The terminalnitrogen is transferred to a secondary amine nitrogen after a succinicacid moiety was attached. The remaining bonds on the nitrogen having asuccinic acid group are preferably filled by hydrogens or alkyl oralkylene groups (linear, branched or cyclic including cyclic structuresjoining more than one nitrogen atom or more than one bond of a singlenitrogen atom, preferably linear) or such groups having ether orthioether linkages, all of preferably from 1 to 10 carbon atoms, morepreferably from 1 to 6, most preferably from 1 to 3 carbon atoms, butmost preferably hydrogen. Generally the nitrogen atoms are linked byalkylene groups, each of from 2 to 12 carbon atoms, preferably from 2 to10 carbon atoms, more preferably from 2 to 8, and most preferably from 2to 6 carbon atoms, namely ethylene, propylene, butylene, pentylene orhexylene. The polyamino monosuccinic acid compound preferably has atleast about 6 carbon atoms and preferably has at most about 50, morepreferably at most about 40, and most preferably at most about 30 carbonatoms. Polyamino monosuccinic acids useful in the practice of theinvention are unsubstituted (preferably) or inertly substituted asdescribed above for polyamino disuccinic acid compounds.

Preferred polyamino monosuccinic acids include ethylenediaminemonosuccinic acid, diethylenetriamine monosuccinic acid,triethylenetetraamine monosuccinic acid, 1,6-hexamethylenediaminemonosuccinic acid, tetraethylenepentamine monosuccinic acid, 2hydroxypropylene-1,3-diamine monosuccinic acid, 1,2-propylenediaminemonosuccinic acid, 1,3-propylenediamine monosuccinic acid,ciscyclohexanediamine monosuccinic acid, transcyclohexanediaminemonosuccinic acid and ethylenebis(oxyethylenenitrilo) monosuccinic acid.The preferred polyamino monosuccinic acid is ethylenediaminemonosuccinic acid.

Such polyamino monosuccinic acids can be prepared for instance, by theprocess of Bersworth et al. in U.S. Pat. No. 2,761,874, the disclosureof which is incorporated herein by reference, and as disclosed in Jpn.Kokai Tokkyo Koho JP 57,116,031. In general, Bersworth et al. disclosereacting alkylene diamines and dialkylene triamines under mildconditions with maleic acid esters under mild conditions (in an alcohol)to yield amino derivatives of N-alkyl substituted aspartic acid. Thereaction yields a mixture of the R and S isomers.

In one embodiment, when the solution contains a mixture of a polyaminodisuccinic acid and a polyamino monosuccinic acid, it is preferred thatthe polyamino substituent of the polyamino disuccinic acid and thepolyamino monosuccinic acid are the same. Thus by way of example, if thepolyamino disuccinic acid is ethylenediamine-N,N′-disuccinic acid, thepolyamine monosuccinic acid is ethylenediamine monosuccinic acid.

In a preferred embodiment, ethylenediamine-N,N′-disuccinic acid (EDDS)is used as complexing agent. EDDS is a preferred complexing agentbecause of its high biodegradability. EDDS is moreover preferred becausea very efficient increase in copper deposition rates can be reached.This effect is also observed when other complexing agents, in additionto EDDS, are present in the bath. Before the present invention was made,it has been observed that increase of metal deposition rate leads toincreased roughness of metal surfaces. In the present invention a highcopper deposition rate and a copper surface with low roughness areobtained.

The increase in deposition rate is proportional to the concentration ofpolyamino disuccinic acid and/or polyamino monosuccinic acid in thebath, particularly in case of EDDS. Thus, the deposition rate can becontrolled by concentration of polyamino disuccinic acid and/orpolyamino monosuccinic acid. This allows an easier integration of theelectroless copper deposition process in already existing processes formanufacture of printed circuit boards (PCB) or displays, and an easieradaptation to already existing production plants and their technique.

The solution of the present invention is free of toxic co-metals. Thesolution of the present invention is especially free of nickel. Nickelforms a more stable complex with the complexing agents used herein thancopper. It therefore reduces copper complexation and negatively affectsor impedes copper deposition. Moreover, the presence of nickel in thebath would lead to unwanted nickel deposition, which has to be avoidedespecially in display production.

The term “EDDS” includes racemic EDDS or optically active isomersthereof, such as (S,S)-EDDS, and salts and derivatives thereof.Preferably the term means (S,S)-EDDS or salts thereof. EDDS may beprepared by the process of PCT/GB94/02397. In the solution,ethylenediaminedisuccinic acid and ethylenediaminedisuccinate-ions maybe present, depending on the pH of the solution.

In one embodiment of a solution of the present invention, the molarratio of the complexing agents, related to the total molar amount of allcomplexing agents, to copper ions is in the range of 1:1 to 10:1,preferably 1:1 to 8:1, more preferably 2:1 to 8:1, more preferably 2:1to 5:1, even more preferably 2:1 to 4:1. In the examples the amount ofcomplexing agents is also given as equivalents. One equivalent is theamount of a complexing agent which completely complexes a given amountof copper ions. In the case of EDDS, EDTA, HEDTA and Quadrol oneequivalent of complexing agent corresponds to a molar ratio ofcomplexing agent to copper ions of 1:1. In the case of EDDS, EDTA, HEDTAand Quadrol a molar ratio of 1:1 to 10:1 of complexing agent(s) tocopper ions means 1 to 10 equivalents of complexing agent(s) related tocopper.

Less complexing agent leads to instability of the bath or depositiondoes not start. More complexing agent in relation to copper leads to ahigh density of the bath, which also leads to reduced lifetime andinstability of the bath. Using these ranges leads to a beneficialcombination of high copper deposition rate and low roughness.

In another embodiment, the molar ratio of the complexing agents, whichmeans the total amount of all complexing agents, to copper ions is inthe range of 3:1 to 8:1, more preferably 3:1 to 5:1, even morepreferably 3:1 to 4:1. Using these ranges leads to a particularlybeneficial combination of high copper deposition rate and low roughness.A very reproducible performance, a very reproducible copper deposition,and copper layers with very uniform thickness can be obtained.

In one embodiment, the molar ratio of complexing agent i) to complexingagent(s) ii), related to the total molar amount of all complexingagent(s) ii), ranges from 1:0.1 to 1:30, 1:0.5 to 1:25, 1:1 to 1:20,preferably from 1:1 to 1:15, more preferably from 1:1 to 1:10 and mostpreferably from 1:1 to 1:8. Using these ranges leads to a beneficialcombination of high copper deposition rate and low roughness.

A specific combination of complexing agents is a combination comprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid        (HEDTA),

A further specific combination of complexing agents is a combinationcomprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) ethylenediamine tetraacetic acid (EDTA).

A further specific combination of complexing agents is a combinationcomprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol)

In one embodiment, the electroless aqueous copper plating solutioncomprises, as complexing agents, a combination comprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid        (HEDTA), and    -   iii) N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine        (Quadrol).

In a further embodiment, the electroless aqueous copper plating solutioncomprises, as complexing agents, a combination comprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid        (HEDTA), and    -   iii) ethylenediamine tetraacetic acid (EDTA)

In a still further embodiment, the electroless aqueous copper platingsolution comprises, as complexing agents, a combination comprising

-   -   i) at least one polyamino disuccinic acid, or at least one        polyamino monosuccinic acid, or a mixture of at least one        polyamino disuccinic acid and at least one polyamino        monosuccinic acid, and    -   ii) N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine        (Quadrol), and    -   iii) ethylenediamine tetraacetic acid (EDTA)

In a still further embodiment, at least one polyamino disuccinic acid,or at least one polyamino monosuccinic acid, or a mixture of at leastone polyamino disuccinic acid and at least one polyamino monosuccinicacid (component i) is combined with EDTA, HEDTA and Quadrol.

A beneficial complexing agent i), in all embodiments, is EDDS.

The solution of the invention in one embodiment contains following kindsof ingredients in following concentrations:

Copper ions: 1-5 g/l, corresponding to 0.016-0.079 mol/l, preferably2.0-3.0 g/l

Reducing agent: 0.027-0.270 mol/l, preferably glyoxylic acid: 2-20 g/l,or formaldehyde: 0.8-8.5 g/l.

Complexing agents (total amount of all complexing agents): 5-50 g/l,more preferably 20-40 g/l.

The solution of the present invention may comprise—and does notnecessarily comprise—further components, as for example stabilizers,surfactants, additives, as rate controlling additives, grain refiningadditives, pH buffers, pH adjusters, and enhancers. Such furthercomponents are for example described in following documents, which areincorporated by reference in their entirety: U.S. Pat. No. 4,617,205(particularly disclosure in col. 6, I. 17—col. 7, I. 25), U.S. Pat. No.7,220,296 (particularly col. 4, I. 63—col. 6, I. 26), US 2008/0223253(cf. particularly paragraphs 0033 and 0038).

Stabilizing agents, also referred to as stabilizers, are compounds thatstabilize the electroless plating solution against unwanted outplatingin the bulk solution. The term “outplating” means unwanted and/oruncontrolled deposition of copper, for example on the bottom of areaction vessel or on other surfaces. Stabilizing function can forexample be accomplished by substances acting as catalyst poison (forexample sulfur or other chalcogenide containing compounds) or bycompounds forming copper(I)-complexes, thus inhibiting the formation ofcopper(I)oxide.

Suitable stabilizers are, without limitation, dipyridyls(2,2′-dipyridyl, 4,4′dipyridyl), phenanthroline, mercapto-benzothiazole,thio-urea or its derivatives, cyanides like NaCN, KCN, K₄[Fe(CN)₆],Na₂S₂O₃, K₂S₂O₃, thiocyanates, iodides, ethanolamines, polymers likepolyacrylamides, polyacrylates, polyethylene glycols, or polypropyleneglycols and their co-polymers.

In another aspect, the present invention relates to a process forelectroless copper plating, the process comprising contacting asubstrate with an electroless copper plating solution as describedabove. In the process, a copper layer is formed on the substrate,preferably with a roughness of 5-40 nm, expressed as root-mean-squareroughness parameter (RMS).

For example, the substrate may be dipped or immersed in the solution ofthe invention. In the process a whole surface of a substrate may beplated with copper, or only selected portions.

It is preferred that the solution be agitated during use. In particular,work- and/or solution-agitation may be used.

The process will be carried out for a sufficient time to yield a depositof the thickness required, which in turn will depend on the particularapplication.

One envisaged application of the process is the preparation of printedcircuit boards. The electroless deposition of copper according to theprocess of the invention can particularly be used for thethrough-plating of holes, surfaces, trenches, blind micro vias inprinted circuit boards. Double sided or multilayer boards (rigid orflexible) may be plated by means of the present invention.

The process of the invention may be useful in providing electrolesscopper deposits with a thickness in the range of 0.05 to 10 μm.Thickness of copper layer is determined with white light interferometry,as described in the examples.

Substrates that are generally used for printed circuit board manufactureare most frequently epoxy resins or epoxy glass composites. But othersubstances, notably phenolic resins, polytetrafluoroethylene (PTFE),polyimides polyphenyleneoxides, BT (bismaleintriazine)-resins, cyanateesters and polysulphones can be used.

Aside from the application of the process in the production of printedcircuit boards, it may be found to be useful in plating substrates madefrom glass, ceramic or plastics, as for example ABS, polycarbonate,polyimide or polyethylene terephthalate.

In another embodiment of the process, the substrate is a substrate madefrom glass, ceramic or plastics, preferably with a large surface area. Alarge surface area means preferably an area of at least one m², morepreferably at least 3 m², still more preferably at least 5 m². A largesurface area means in another embodiment preferably an area of 1 m² to 9m², more preferably 3 m² to 9 m², still more preferably 5 m² to 9 m².The substrate has preferably a smooth surface. The term smooth meanspreferably a roughness (Sq or RMS) of a few nanometers. Preferably theroughness is 5-30 nm, expressed as root-mean-square roughness parameter(RMS). Explanations of the method for roughness measurement and of “Sq”and “RMS” are given in the examples.

In a special embodiment, the substrate is a glass substrate, preferablya glass panel. Said glass substrates, especially glass panels can beused for application in TFT displays, such as liquid crystal displays.Thus, the glass substrate is particularly such one that fulfils thespecifications as used in display production, as for example thicknessand smoothness. A preferred glass is free from alkali, such as alkalifree boro-silicate-glass.

Glass substrates may be pretreated before the process of the inventionis carried out, for example with metal seeds, as further explainedbelow.

In one embodiment of the process of the present invention, the processis carried out at a temperature in the range of 20-60° C., preferably30-55° C. It has been shown in the present invention that when at leastone polyamino disuccinic acid, or at least one polyamino monosuccinicacid, or a mixture of at least one polyamino disuccinic acid and atleast one polyamino monosuccinic acid, particularly EDDS, is used ascomplexing agent, in combination with another complexing agent, copperdeposition can be done at lower temperatures than in absence of thiscomponent. Even though the temperature is lower, the deposition rate ishigher than with a bath that does not contain at least one polyaminodisuccinic acid, or at least one polyamino monosuccinic acid, or amixture of at least one polyamino disuccinic acid and at least onepolyamino monosuccinic acid, particularly EDDS.

The substrate, i.e. the surfaces of the substrate that are to be platedwith copper, particularly non-metallic surfaces, may be pretreated bymeans within the skill in the art (as for example described in U.S. Pat.No. 4,617,205, col 8) to make it/them more receptive or autocatalyticfor copper deposition. All or selected portions of a surface may bepretreated. A pretreatment is, however, not necessary in every case anddepends on the kind of substrate and surface. Within the pretreatment,it is possible to sensitise substrates prior to the deposition ofelectroless copper on them. This may be achieved by the adsorption of acatalysing metal (such as a noble metal, for example palladium) onto thesurface of the substrate.

A pretreatment process strongly depends on parameters as the substrate,the desired application, and the desired properties of the coppersurface.

An exemplary and non-limiting pretreatment process, especially forprinting circuit board laminates and other suitable substrates, maycomprise one or more of the following steps

-   -   a) optionally cleaning and conditioning the substrate to        increase adsorption. With a cleaner, organics and other residues        are removed. It may also contain additional substances        (conditioners) that prepare the surface for the following        activation steps, i.e. enhance the adsorption of the catalyst        and lead to a more uniformly activated surface,    -   b) etching, to remove oxides from the surface of the copper,        especially from inner layers in holes. This may be done by        persulphate or peroxide based etching systems,    -   c) contacting with a pre-dip solution, such as a hydrochloric        acid solution or sulfuric acid solution, optionally with an        alkali metal salt, such as sodium chloride, also in the pre-dip        solution,    -   d) contacting with an activator solution, that contains        colloidal or ionic catalysing metal, such as a noble metal,        preferably palladium, causing the surface to become catalytic.        -   The pre-dip in step c) serves to protect the activator from            drag-in and contaminations, and optionally, particularly if            the activator contains ionic catalysing metal,    -   e) contacting with a reducer, wherein the metal ions of an ionic        activator are reduced to elemental metal.    -   or, if the activator contains colloidal catalysing metal,    -   f) contacting with an accelerator, wherein components of the        colloid, for example a protective colloid, is removed from the        catalysing metal.

In another kind of pretreatment process a permanganate etching step isemployed. The so-called Desmear process is a multi-stage process, thesteps of which are a swelling step, a permanganate etching step and areduction step. The sweller used in the swelling step is made of amixture of organic solvents. During this step drill smear and otherimpurities are removed from the surfaces of the substrate. A hightemperature of 60-80° C. promotes the infiltration of the sweller whichleads to a swelled surface. Therefore a stronger attack of thesubsequently applied permanganate solution is possible during thepermanganate etching step. Afterwards the reduction solution of thereduction step removes the manganese dioxides produced during thepermanganate step from the surfaces. The reduction solution contains areducing agent and optionally a conditioner.

The desmear process may be combined with the above described steps. Thedesmear process may be performed before step a) of the above describedpretreatment process or the desmear process may be performed instead ofsteps a) and b) of the above described pretreatment process.

In a pretreatment process which is particularly suitable inmetallization for display applications and in metallization of glasssubstrates, a surface is only contacted with a pre-dip solution and anactivator solution and then with the solution of the invention.Contacting with a cleaning solution and an adhesion enhancer before thepre-dip step are optional steps that can be carried out in advance.

Still another process, which is often used for glass substrates, may becarried out with following steps before copper plating: A glass surfacethat is to be plated exhibits metal seed layers. The metal seed layersmay be brought onto the surface by sputtering techniques. Exemplaryseeds are layers composed of copper, molybdenum, titanium or a mixturethereof. Said pretreated glass surface is contacted with an activatorsolution that contains ionic catalysing metal, such as a noble metal,preferably palladium, causing the surface to become catalytic. The ioniccatalysing metal is reduced onto the surface by the seed metal. In thisprocess, addition of a further reducer may be omitted. This process isespecially used in copper plating of glass substrates for displayapplications.

The exemplary pretreatment processes, or single steps thereof, may becombined to alternative pretreatment processes, if found necessary.

In a further aspect, the present invention relates to the use of theelectroless copper plating solution as described above for the platingof printed circuit boards, wafers, Integrated circuit substrates, moldedinterconnect device (MID) components, displays, such as liquid crystalor plasma displays, particularly displays for electronic devices or TVs,display components, or plastic parts, such as plastic parts forfunctional or decorative purposes.

DESCRIPTION OF FIGURES

FIG. 1 effect of a combination of polyamino disuccinic acid with onefurther complexing agent on copper thickness in a plating process

FIG. 2a-c Combination of polyamino disuccinic acid with two furthercomplexing agents; effect of polyamino disuccinic acid addition oncopper deposition rate and roughness of copper layer in a platingprocess

FIG. 3 Beaker test, combination of polyamino disuccinic acid with twofurther complexing agents; effect of polyamino disuccinic acid additionon copper deposition rate of copper in a plating process

FIG. 4 Horizontal line experiment, combination of polyamino disuccinicacid with two further complexing agents; effect of polyamino disuccinicacid addition on copper deposition rate of copper in a plating process

FIG. 5 Comparative Example, effect of varying amounts of two othercomplexing agents, without addition of polyamino disuccinic acid

FIG. 6 Results of the variation of the ratio of two complexing agents

FIG. 7 Results of the variation of the ratio of copper to totalcomplexing agents

The invention is now described in further detail by the followingexamples. These examples are set forth to illustrate the presentinvention, but should not be construed as limiting the presentinvention.

Method of Roughness Measurement:

An Optical profilometer/White light interferometer, Model MIC-520, ofATOS GmbH (Germany) was used to measure the thickness (Height differencebetween base plane and plated pattern) and surface roughness ofelectrolessly plated copper layers. White light interferometry is anoptical microscopy method known to persons skilled in the art whichprojects the target area of a sample onto a CCD camera. Usinginterference objectives equipped with an internal beam splitter, ahigh-precision reference mirror is projected onto the CCD camera aswell. Due to the overlay of both images, a spatially resolvedinterferogram is created which reflects the height differences betweenthe very flat reference mirror and the sample of interest. In order toimage samples with large height distribution a vertical scan scheme isused, i.e. interferograms of the area of interest are imaged as a serieswithin a range of different sample-objective distances. From these dataa full three dimensional image is compiled. Using this methodtopographic images in the range of 60 μm×60 μm to 1.2 mm×1.2 mm can berecorded with a vertical resolution in the range of a few nm.

The topographic data are used to calculate surface roughness expressedas the root-mean-square roughness parameter, abbreviated as Rq or RMS onsurface profiles (profile roughness parameter) and abbreviated as Sq onsurface topographies (areal roughness parameter). The meaning of Rq isidentical to the meaning of RMS. Rq, or RMS, has the meaning as definedin DIN EN ISO 4287 (German and English version of 1998, Chapter 4.2.2)and Sq has the meaning as defined in ISO 25178-2 of April 2012 (Chapter4.1.1).

In addition the topographic data are used to calculate the thickness ofthe plated copper layers as height difference between the substratesurface (base plane) and the surface of the plated metal pattern. Forcalculating topographic images, layer thickness and surface roughnessthe Optical profilometer/White light interferometer, Model MIC-520, ofATOS GmbH (Germany) was equipped with the computer software Micromap123, version 4.0, by Micromap Corporation.

The mode of measurement was Focus 560 M. The topographic images weremeasured with an objective lens with 10 times magnification and anocular with 2 times magnification. The topographic images were recordedin the range of 312 μm×312 μm and consist of 480×480 points.

EXAMPLE 1 Combination of Polyamino Disuccinic Acid with a FurtherComplexing Agent

Substrate: Alkali-Free Borosilicate Glass, Thickness 0.7 mm, SputteredSeed Layer of Copper.

Pre Treatment:

1. alkaline cleaner 40° C./1 min

2. rinsing with H₂O

3. sulfuric acid pre dip solution, room temperature (RT)/20 sec

4. ionic Pd-activator (exchange-reaction between Cu and Pd) RT/2 min

5. rinsing with H₂O

Electroless copper plating solutions were manufactured. As complexingagents combinations of EDDS/EDTA, and EDDS/HEDTA were employed. EDDS wasadded in amounts of 0 g/l, 4.5 g/l and 6.8 g/l, respectively. Cu2₊ ionswere added as CuSO4*6H2O, the enhancer H2PO2⁻ was added as NaH2PO2*2H2O, and as stabilizer a mixture of cyanide and sulphur compoundswas added. The pH of the baths were 13.2 at 21° C.

EDDS as only complexing agent, in different amounts, was employed ascomparative example.

Substrates were contacted with the respective plating solutions asdescribed above at 55° C. for 10 min each. The samples of deposited Culayers were analysed according to the described method in measuring mode“Focus 560 M”. The results are shown in the following tables 1 to 4.FIG. 1 shows a chart of the results obtained.

TABLE 1 Combination of EDDS/EDTA 0 ml/l EDDS (comp.) 20 ml/l EDDS 30ml/l EDDS EDTA 23.1 g/l 23.1 g/l 23.1 g/l EDTA in Equivalents 2.0 2.02.0 Cu²⁺ 2.5 g/l 2.5 g/l 2.5 g/l H₂PO₂ ⁻ 9.1 g/l 9.1 g/l 9.1 g/l KOH 10g/l 10 g/l 10 g/l Glyoxylic Acid 4.5 g/l 4.5 g/l 4.5 g/l Stabilizer 10mg/l 10 mg/l 10 mg/l EDDS 0 g/l 4.5 g/l 6.8 g/l EDDS in Equivalents 00.4 0.6 Temperature [° C.] 55 55 55 Cu thickness [μm] 0.50 1.13 1.47

TABLE 3 Combination of EDDS/HEDTA 0 ml/l EDDS (comp.) 20 ml/l EDDS 30ml/l EDDS HEDTA 22.1 g/l 22.1 g/l 22.1 g/l HEDTA in Equivalents 2.0 2.02.0 Cu²⁺ 2.5 g/l 2.5 g/l 2.5 g/l H₂PO₂ ⁻ 9.1 g/l 9.1 g/l 9.1 g/l KOH 10g/l 10 g/l 10 g/l Glyoxylic Acid 4.5 g/l 4.5 g/l 4.5 g/l Stabilizer 10mg/l 10 mg/l 10 mg/l EDDS 0 g/l 4.5 g/l 6.8 g/l EDDS in Equivalents 00.4 0.6 Temperature [° C.] 55 55 55 Cu thickness [μm] 1.31 2.28 3.18

TABLE 4 Comparative Example, only EDDS in different amounts Comp.Example 0 ml/l EDDS (comp.) 20 ml/l EDDS 30 ml/l EDDS EDDS 28.4 g/l 28.4g/l 28.4 g/l EDDS in Equivalents 2.0 2.0 2.0 Cu²⁺ 2.5 g/l 2.5 g/l 2.5g/l H₂PO₂ ⁻ 9.1 g/l 9.1 g/l 9.1 g/l KOH 10 g/l 10 g/l 10 g/l GlyoxylicAcid 4.5 g/l 4.5 g/l 4.5 g/l Stabilizer 10 mg/l 10 mg/l 10 mg/l EDDS 0g/l 4.5 g/l 6.8 g/l EDDS in Equivalents 0 0.4 0.6 Temperature [° C.] 5555 55 Cu thickness [μm] 3.34 3.67 4.09

A combination of EDDS/EDTA (table 1, FIG. 1 left) leads to increasedcopper thicknesses in comparison to EDTA alone, when the same processtime is chosen. Copper thickness can be increased by a factor ofapproximately 3, from 0.5 μm (no EDDS) to 1.47 μm (6.8 g/l EDDS).

A combination of EDDS/HEDTA (table 3, FIG. 1 middle) leads to increasedcopper thicknesses in comparison to HEDTA alone, when the same processtime is chosen. Copper thickness can be increased by a factor ofapproximately 2.4, from 1.31 μm (no EDDS) to 3.18 μm (6.8 g/l EDDS).

Less strong influence on copper thickness is observed when increasedamounts of EDDS, as only complexing agent, are employed (FIG. 1, rightside; table 4). 28.4 g/l EDDS are employed as basic amount and amountsof 0 g/l, 4.5 g/l and 6.8 g/l were added, as in the other examples.Copper thickness is increased by a factor of approximately 1.2 only,from 3.34 μm (0 g/l additional EDDS) to 4.09 μm (6.8 g/l additionalEDDS).

EXAMPLE 2 Combination of Polyamino Disuccinic Acid with Two FurtherComplexing Agents (HEDTA and Quadrol)

The same substrates, pre-treatment procedures, components of the bathand pH were used as in Example 1. As complexing agents EDDS, HEDTA andN,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine were used.N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine is hereinafterabbreviated as “Quadrol”, which is a trademark of BASF company.

Deposition rate was determined from the thickness of copper layer,determined with white light interferometry, and the deposition time.

The experimental set-up and results are shown in table 5. In experimentNo. 1, 2 and 3, different amounts glyoxylic acid were employed. Theresults as to deposition rate and roughness (Sq) are also shown in FIG.2a (experiments No. 1A and 1B from table 5), FIG. 2b (experiments No. 2Aand 2B from table 5) and FIG. 2c (experiments No. 3A and 3B from table5). Dwell time means the time of contacting the substrates with theelectroless copper plating solutions.

The results show that addition of EDDS increases deposition rate andreduces at the same time roughness of the deposited copper layers.

TABLE 5 Combination of polyamino disuccinic acid with two furthercomplexing agents (HEDTA and Quadrol) Cu Dwell deposition Glyoxylic EDDSThickness Sq Time rate Cu²⁺ Acid KOH H₂PO₂ ⁻ HEDTA Quadrol Stabilizer TNo. Content [μm] [nm] [min] [μm/10 min] [g/l] [g/l] [g/l] [g/l] [g/l][g/l] [mg/l] [° C.] 1A 0 ml/l 1.88 39 22 0.85 2.5 3.4 8 9.1 32.1 1.7 1052 1B 20 ml/l = 1.96 27 20 0.98 2.5 3.4 8 9.1 32.1 1.7 10 50 4.5 g/l =0.4 Eq 2A 0 ml/l 1.02 36 22 0.46 2.5 4.5 10 9.1 32.1 1.7 10 55 2B 20ml/l = 0.98 20 10 0.98 2.5 3.4 8 9.1 32.1 1.7 10 50 4.5 g/l = 0.4 Eq 3A0 ml/l 1.01 42 22 0.46 2.5 4.5 10 9.1 32.1 1.7 10 55 3B 40 ml/l = 0.9625 9.5 1.01 2.5 4.2 8 9.1 32.1 1.7 10 49 9.1 g/l = 0.8 Eq 2.9 Eq 0.15 EqEq = Equivalent

EXAMPLE 3 Combination of Polyamino Disuccinic Acid with Two FurtherComplexing Agents (HEDTA and Quadrol)

The same substrates, pre-treatment procedures, components of the bathand pH were used as in Example 1

3.1 Beaker Test

Experiments were done in a beaker. In Table 6, bath compositions andresults of experiments are shown.

In experiments No. 4 and No. 5 three equivalents HEDTA/Quadrol perequivalent copper were used. In experiment No. 6 two equivalents EDDSper equivalent copper were added. FIG. 3 shows the effect of complexingagents on deposition rate. Addition of EDDS to the mixture of complexingagents HEDTA and Quadrol leads to an increase in deposition rate(experiment No. 6). As shown in the comparative example 3.3 below,further addition of HEDTA/Quadrol does not lead to an increase indeposition rate.

TABLE 6 Beaker Test 0 ml/l EDDS 0 ml/l EDDS 65 ml/l EDDS Experiment Name4 5 6 HEDTA 32.1 g/l 32.1 g/l 32.1 g/l HEDTA in Equivalents 2.9 2.9 2.9Quadrol 2.3 g/l 2.3 g/l 2.3 g/l Quadrol in Equivalents 0.2 0.2 0.2 Cu²⁺2.5 g/l 2.5 g/l 2.5 g/l H₂PO₂ ⁻ 9.1 g/l 9.1 g/l 9.1 g/l KOH 10 g/l 10g/l 10 g/l Glyoxylic Acid 4.5 g/l 4.5 g/l 4.5 g/l Stabilizer 10 mg/l 10mg/l 10 mg/l EDDS 0 g/l 0 g/l 23.3 g/l EDDS in Equivalents 0 0 2.0Temperature [° C.] 55 55 55 Cu Deposition Rate [μm/ 0.55 0.43 1.17 10min]3.2 Horizontal Line Tests

Experiments were done in a Horizontal conveyorized equipment. In Table7, bath compositions and results of experiments are shown.

In experiment No. 7 three equivalents HEDTA/Quadrol per equivalentcopper were used. In experiment No. 8, EDDS was added as additionalcomplexing agent. FIG. 4 shows the effect of complexing agents ondeposition rate. Addition of EDDS to the mixture of complexing agentsHEDTA and Quadrol leads to an increase in deposition rate. As shown inthe comparative example 3.3 below, further addition of HEDTA/Quadroldoes not lead to an increase in deposition rate.

TABLE 7 Horizontal Line setup 0 ml/l EDDS 40 ml/l EDDS Experiment Name 78 HEDTA 32.1 g/l 32.1 g/l HEDTA in Equivalents 2.9 2.9 Quadrol 2.3 g/l2.3 g/l Quadrol in Equivalents 0.2 0.2 Cu²⁺ 2.5 g/l 2.5 g/l H₂PO₂ ⁻ 9.1g/l 9.1 g/l KOH 10 g/l 10 g/l Glyoxylic Acid 4.5 g/l 4.5 g/l Stabilizer10 mg/l 10 mg/l EDDS 0 g/l 14.3 g/l EDDS in Equivalents 0 1.2Temperature [° C.] 55 55 Average Cu Deposition Rate 0.62 0.95 [μm/10min]3.3 Comparative Example, HEDTA and Quadrol Only

Experiments were done in a Horizontal Line setup. In Table 8, bathcompositions and results of experiments are shown.

In experiments No. 9 and No. 10, three equivalents HEDTA/Quadrol perequivalent copper were used.

Starting in experiment No. 11, increasing amounts of further HEDTA andQuadrol were added, up to six equivalents HEDTA/Quadrol per equivalentcopper in total (experiment No. 16).

FIG. 5 shows the effect of complexing agents on deposition rate. As canbe seen from the results, an increase of HEDTA/Quadrol concentrationdoes not lead to increased deposition rate. On the contrary, a trend tolower deposition rates can be observed, when further HEDTA/Quadrol isadded.

TABLE 8 Horizontal Line setup Experiment Name 9 10 11 12 13 14 15 16HEDTA 32.1 g/l 32.1 g/l 32.1 g/l 32.1 g/l 32.1 g/l 32.1 g/l 32.1 g/l32.1 g/l Quadrol 2.3 g/l 2.3 g/l 2.3 g/l 2.3 g/l 2.3 g/l 2.3 g/l 2.3 g/l2.3 g/l Cu²⁺ 2.5 g/l 2.5 g/l 2.5 g/l 2.5 g/l 2.5 g/l 2.5 g/l 2.5 g/l 2.5g/l H₂PO₂ ⁻ 9.1 g/l 9.1 g/l 9.1 g/l 9.1 g/l 9.1 g/l 9.1 g/l 9.1 g/l 9.1g/l KOH 10 g/l 10 g/l 10 g/l 10 g/l 10 g/l 10 g/l 10 g/l 10 g/lGlyoxylic Acid 4.5 g/l 4.5 g/l 4.5 g/l 4.5 g/l 4.5 g/l 4.5 g/l 4.5 g/l4.5 g/l Stabilizer 10 mg/l 10 mg/l 10 mg/l 10 mg/l 10 mg/l 10 mg/l 10mg/l 10 mg/l HEDTA (additional) 0 g/l 0 g/l 6.6 g/l 12.1 g/l 17.6 g/l23.1 g/l 28.6 g/l 34.1 g/l Quadrol (additional) 0 g/l 0 g/l 0.5 g/l 0.9g/l 1.3 g/l 1.7 g/l 2.1 g/l 2.4 g/l Temperature [° C.] 55 55 55 55 55 5555 55 Cu Deposition Rate 0.41 0.30 0.28 0.28 0.28 0.28 0.27 0.24 [μm/10min]

EXAMPLE 4 Variation of the Ratio of Two Complexing Agents (EDDS andHEDTA)

Experiments were conducted as in Example 1.

TABLE 9 Ratio of total complexing agents to Cu Ratio of complexingagents (i + ii):Cu (i EDDS):(ii HEDTA) 3.1:1  1:31 3.1:1 1:8 3.1:1 1:43.1:1   1:0.05

Bath composition Cu²⁺ 2.5 g/l KOH 8 g/l Stabilizer 2 mg/l glyoxylic acid4.6 g/l temperature 45° C.

Deposition time: 10 min

Further details of experiments and results are given in table 10.

TABLE 10 ratio of Seed Cu Thickness [μm] Roughness EDDS: ComplexingLayer standard Sq [nm] HEDTA agents:Cu [μm] average deviation averageComment  1:31 3.1:1 0.344 0.196 0.01 7 1:8 3.1:1 0.344 0.264 0.01 8 1:43.1:1 0.344 0.266 0.04 8   1:0.05 3.1:1 0.344 0.081 0.05 42 inhomogenousdeposit 1:4 0.5:1 0.344 −0.001 0.01 6 bath was close to instable, H₂formation 1:4 3.1:1 0.344 0.266 0.04 8 1:4  11:1 0.344 0.921 0.06 54bath was close to instable, H₂ formation, wild growth

The results of the variation of the ratio of two complexing agents (EDDSand HEDTA) are shown in FIG. 6. Highest copper thickness is reached atratio EDDS to HEDTA of 1:8 and 1:4. At ratio of 1:0.05 thickness ofcopper decreases strongly and roughness increases strongly.

EXAMPLE 5 Variation of the Ratio of Copper to Total Complexing Agents(EDDS and HEDTA)

Experiments were conducted as in Example 1.

TABLE 11 Ratio of total complexing agents to Cu Ratio of complexingagents (i + ii):Cu (i EDDS):(ii HEDTA) 0.5:1 1:4 3.1:1 1:4  11:1 1:4

Bath composition Cu²⁺ 2.5 g/l KOH 8 g/l Stabilizer 2 mg/l glyoxylic acid4.6 g/l temperature 45° C.

Deposition time: 10 min

Further details of experiments and results are given in table 10.

The results of the variation of the ratio of copper to total complexingagents (EDDS and HEDTA) are shown in FIG. 7. Copper thickness but alsoroughness increase with increasing ratio of total complexing agents tocopper. Copper Baths with a ratio of total complexing agents to Cu of0.5:1- and 11:1 were unstable, the bath with a ratio of 3.1:1 wasstable.

The invention claimed is:
 1. An electroless aqueous copper platingsolution comprising: a source of copper ions; a reducing agent or asource of a reducing agent; and a combination of complexing agents, thecombination comprising: i) polyamino disuccinic acid, polyaminomonosuccinic acid, or combinations thereof, and ii) one or more of acompound which is selected from the group consisting of ethylenediaminetetraacetic acid, N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triaceticacid, and N,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine.
 2. Theelectroless aqueous copper plating solution according to claim 1,wherein the combination of complexing agents comprises:N′-(2-Hydroxyethyl)-ethylenediamine-N,N,N′-triacetic acid andN,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine.
 3. The electrolessaqueous copper plating solution according to claim 1, wherein thecombination of complexing agents comprises polyamino disuccinic acid. 4.The electroless aqueous copper plating solution according to claim 1,wherein the combination of complexing agents comprisesethylenediaminedisuccinic acid.
 5. The electroless aqueous copperplating solution according to claim 1, wherein the molar ratio of thecombination of complexing agents to copper ions is in the range of 1:1to 10:1.
 6. The electroless aqueous copper plating solution according toclaim 1, wherein the reducing agent is one or more of glyoxylic acid andformaldehyde.
 7. The electroless aqueous copper plating solutionaccording to claim 1, wherein the solution further comprises astabilizing agent.
 8. The electroless aqueous copper plating solutionaccording to claim 7, wherein the stabilizing agent is a dipyridyl,phenanthroline, mercapto-benzothiazole, thio-urea or derivative,cyanide, thiocyanate, iodide, ethanolamine, or a polymer.
 9. Theelectroless aqueous copper plating solution according to claim 1,wherein the solution further comprises a source of hydroxide ions.
 10. Amethod for electroless copper plating, the method comprising contactinga substrate with an electroless copper plating solution according toclaim
 1. 11. The method according to claim 10, wherein the substrate ismade from glass, ceramic or plastics.
 12. The method according to claim10, wherein the substrate is glass.
 13. The method according to claim10, wherein a copper layer having a roughness of 5-40 nm, expressed asroot-mean-square roughness parameter (RMS), is formed on the substrate.14. A method of plating a printed circuit board, integrated circuitsubstrate, wafer, molded interconnect device, display, display componentor plastic part comprising contacting the printed circuit boards,integrated circuit substrates, wafers, molded interconnect devices,displays, display components or plastic parts with the electrolesscopper plating solution according to claim
 1. 15. A method of plating aglass substrate comprising contacting the glass substrates with theelectroless copper plating solution according to claim
 1. 16. The methodaccording to claim 10, wherein the substrate has a surface area of atleast 1 m².
 17. The method according to claim 12, wherein the substrateis a glass panel.
 18. The method according to claim 15, wherein theglass substrate is a glass panel for display.
 19. The electrolessaqueous copper plating solution according to claim 8, wherein thepolymer is a polyacrylamide, polyacrylate, polyethylene glycol,polypropylene glycol, or co-polymer of any of the foregoing.